Process for Recovering Molybdate or Tungstate from Aqueous Solutions

ABSTRACT

Process for recovering molybdate or tungstate from an aqueous solution, in which molybdate or tungstate is bound to a water-insoluble, cationized inorganic carrier material from the aqueous solution at a pH in the range from 2 to 6, the laden carrier material is separated off and the bound molybdate or tungstate is liberated once again into aqueous solution at a pH in the range from 6 to 14. The process is suitable for recovering molybdate or tungstate in the delignification of pulp with hydrogen peroxide in the presence of molybdate or tungstate as catalyst. The recovered molybdate or tungstate can be recycled to the delignification.

CROSS REFERENCE TO RELATED APPLICATIONS

The present application is a division of U.S. Ser. No. 12/431,208, filedon Apr. 28, 2009, which claims the benefit of U.S. provisionalapplication 61/136,865 filed on Oct. 9, 2008 and priority to GermanApplication (DE) 102008040884.0, filed on Jul. 31, 2008 and Europeanapplication (EP) 08103792.1, filed on Apr. 30, 2008. The contents ofthese prior applications are hereby incorporated by reference in theirentirety.

FIELD OF THE INVENTION

The invention relates to a process for recovering molybdate or tungstatefrom aqueous solutions, which is particularly suitable for recoveringmolybdate or tungstate in the molybdate- or tungstate-catalyzeddelignification of pulp by means of hydrogen peroxide.

BACKGROUND OF THE INVENTION

Pulp bleaching is usually carried out using hydrogen peroxide in analkaline medium, since free-radicals are formed in an acidic medium atelevated temperature and these lead to undesirable secondary reactionssuch as degradation of cellulose. However, if a suitable catalyst isused, delignification and bleaching by means of hydrogen peroxide isalso possible under acidic conditions.

U.S. Pat. No. 4,427,490 describes the delignification and bleaching ofkraft pulp by means of hydrogen peroxide in an acidic medium, catalyzedby sodium tungstate or sodium molybdate. In Journal of Pulp and PaperScience, Vol. 18, 1992, pages J108-J114, V. Kubelka describes a processfor the delignification of pulp which has steps for delignification bymeans of oxygen and an intermediate delignification by means of hydrogenperoxide, which is carried out at a pH of 5 using sodium molybdate ascatalyst. In the article, it is proposed that molybdate be recovered ina known manner using anion exchangers.

JP 11 130 762 describes the recovery of tungstate from a reactionmixture from the tungstate-catalyzed oxidation of maleic anhydride withaqueous hydrogen peroxide. The reaction mixture is for this purposepassed over a chelating resin having glucamine substituents and theresin is subsequently washed with aqueous sulphuric acid. The tungstateis then washed out of the chelating resin using aqueous sodiumhydroxide. FR 2 320 946 describes the recovery of tungstate from thesame reaction mixture with a strong-base anion-exchange resin.

JP 2003048716 describes a recovery of molybdate using a chelatingion-exchange resin.

CZ 279 703 describes a multistage recovery process for molybdate, inwhich molybdate is firstly adsorbed on a weak-basestyrene-divinylbenzene ion exchanger and the molybdate is liberated in asubsequent step with aqueous ammonia.

JP 06010089 B proposes a chelating resin containing dithiocarboxylgroups for the recovery of molybdate.

However, the recovery of molybdate or tungstate using ion-exchangecolumns is too complicated and uneconomical for use in pulp bleachingbecause of the steps required for flushing of the ion exchangers. Inaddition, the degradation products of lignin formed in thedelignification of pulp are adsorbed on the ion-exchange resins used inthe prior art, which has an adverse effect on the ion-exchangeproperties of the ion-exchange resin.

In the proceedings of the 93^(rd) annual conference of the PATPTAC 2007,pages A261-A268, R. C. Francis et al. propose precipitation of themolybdate catalyst in the catalyzed delignification of pulp by additionof the cationic surfactant cetyltrimethylammonium bromide and filteringoff the complex formed. The molybdate is recovered from the filtered offcomplex by dissolution in sodium hydroxide solution and extraction ofthe cationic surfactant with a solvent such as isobutanol. However, theprocess proposed has the disadvantage that the complex formed frommolybdate with cetyltrimethylammonium bromide is difficult to filter andthe use of an additional organic solvent is necessary for recovery ofmolybdate.

For this reason, there continues to be a need for a process which issimple to carry out and whereby molybdate or tungstate can be recoveredfrom an aqueous solution to a high degree, with the process beingsuitable for recovering molybdate or tungstate from the aqueoussolutions obtained in the delignification of pulp.

DESCRIPTION OF THE INVENTION

It has now surprisingly been found that this object can be achieved bythe use of a water-insoluble, cationized inorganic carrier material. Thecarrier material according to the invention binds molybdate or tungstatefrom aqueous solutions at a pH in the range from 2 to 6 and liberatesthe bound molybdate or tungstate again in aqueous solution at a pH inthe range from 6 to 14. The carrier material according to the inventioncan also be separated off from the aqueous solution in a simple mannerin both pH ranges by sedimentation, filtration or centrifugation.

The invention accordingly provides a process for recovering molybdate ortungstate from an aqueous solution, which comprises the following steps:

-   a) contacting a molybdate- or tungstate-containing aqueous solution    with a water-insoluble, cationized inorganic carrier material at a    pH in the range from 2 to 6 to give a carrier material laden with    molybdate or tungstate and an aqueous solution depleted in molybdate    or tungstate,-   b) separating the carrier material laden with molybdate or tungstate    from the aqueous solution depleted in molybdate or tungstate,-   c) contacting the carrier material laden with molybdate or tungstate    with an aqueous solution at a pH in the range from 6 to 14 to give a    carrier material depleted in molybdate or tungstate and an aqueous    solution laden with molybdate or tungstate, and-   d) separating the carrier material depleted in molybdate or    tungstate from the aqueous solution laden with molybdate or    tungstate.

For the purposes of the present invention, the terms molybdate andtungstate encompass both mononuclear molybdates and tungstates, e.g.MoO₄ ²⁻ or WO₄ ²⁻, and also polynuclear molybdates and tungstates, e.g.Mo₇O₂₄ ⁶⁻, Mo₈O₂₆ ⁴⁻, HW₆O₂₁ ⁵⁻, W₁₂O₄₁ ¹⁰⁻ or W₁₂O₃₉ ⁶⁻, andheteroatom-containing polynuclear molybdates and tungstates, e.g.PMo₁₂O₄₀ ³⁻, SiMo₁₂O₄₀ ³⁻, PW₁₂O₄₀ ³⁻ or SiW₁₂O₄₀ ³⁻.

In step a) of the process of the invention, the molybdate- ortungstate-containing aqueous solution is brought into contact with thewater-insoluble, cationized inorganic carrier material at a pH in therange from 2 to 6, preferably in the range from 3 to 5, particularlypreferably in the range from 3.5 to 4. The setting of a pH in this rangemakes it possible to achieve virtually complete recovery of molybdate ortungstate from the aqueous solution with a low consumption of agents forregulating the pH. During contacting, the water-insoluble, cationizedinorganic carrier material is preferably dispersed in the molybdate- ortungstate-containing aqueous solution by means of a stirrer or adisperser. Contacting can be carried out at any temperature;temperatures in the range from 0 to 100° C. are suitable.

In step a), the cationized inorganic carrier material is preferably usedin an amount of from 10 to 1000 parts by weight of carrier material perpart by weight of molybdenum or in an amount of from 200 to 10 000 partsby weight of carrier material per part by weight of tungsten in thecontacting with the molybdate- or tungstate-containing aqueous solution.To recover molybdate, particular preference is given to using from 50 to500 parts by weight and in particular from 100 to 300 parts by weight ofcarrier material per part by weight of molybdenum. To recover tungstate,particular preference is given to using from 1000 to 5000 parts byweight and in particular from 2000 to 3000 parts by weight of carriermaterial per part by weight of molybdenum.

In step b) of the process of the invention, the carrier material ladenwith molybdate or tungstate is separated from the aqueous solutiondepleted in molybdate or tungstate. The separation can be carried outusing all solid-liquid separation processes known to those skilled inthe art, for example by sedimentation, filtration or centrifugation. Thecarrier material laden with molybdate or tungstate which has beenseparated off can additionally be washed with water having a pH in therange from 2 to 6 in order to reduce the proportion of adhering organicimpurities.

In step c) of the process of the invention, the carrier material ladenwith molybdate or tungstate which has been separated off in step b) isbrought into contact with an aqueous solution at a pH in the range from6 to 14. In this pH range, molybdate or tungstate is detached once againfrom the support and a carrier material depleted in molybdate ortungstate and an aqueous solution laden with molybdate or tungstate areobtained. The pH is preferably selected in the range from 8 to 12 andparticularly preferably in the range from 9 to 11. Setting of a pH inthese ranges makes it possible for molybdate or tungstate to bevirtually completely detached from the support with a low consumption ofagents for regulating the pH. In the contacting, the carrier materialladen with molybdate or tungstate is preferably dispersed in the aqueoussolution by means of a stirrer or a disperser. Contacting can be carriedout at any temperature; temperatures in the range from 0 to 100° C. aresuitable.

In step d) of the process of the invention, the carrier materialdepleted in molybdate or tungstate is separated from the aqueoussolution laden with molybdate or tungstate. The separation can becarried out using all solid-liquid separation processes known to thoseskilled in the art, for example by sedimentation, filtration orcentrifugation. The carrier material depleted in molybdate or tungstatewhich has been separated off can be additionally washed with an aqueoussolution having a pH in the range from 6 to 14 to complete thedetachment of molybdate or tungstate from the carrier material. Thewashing liquid resulting from washing is preferably combined with theaqueous solution laden with molybdate or tungstate.

The carrier material depleted in molybdate or tungstate which has beenseparated off in step d) can be used again in step a) of the process forrecovering molybdate or tungstate.

In the process of the invention, a water-insoluble, cationized inorganiccarrier material is used for the separation. Suitable cationizedinorganic carrier materials are inorganic carrier materials whosesurface has been modified with positively charged functional groups. Themodification can be carried out, for example, by reaction of the surfacewith reagents which covalently anchor a positively charged functionalgroup on the surface. Suitable water-insoluble, cationized inorganiccarrier materials having covalently anchored positively chargedfunctional groups are, for example, aminosilane-modified precipitated orpyrogenic silicas, which are preferably additionally quaternized on theamino group. As an alternative, the modification can also be effected byion exchange of an inorganic carrier material which is negativelycharged on the surface with a quaternary ammonium salt. The quaternaryammonium salt used for this purpose preferably has at least one nonpolaralkyl radical having from 6 to 24, particularly preferably from 12 to22, carbon atoms in order to prevent detachment of the quaternaryammonium ions from the support in acid conditions.

Preference is given to using a cationized sheet silicate, particularlypreferably a sheet silicate which has been ion-exchanged with aquaternary ammonium salt, as water-insoluble, cationized inorganiccarrier material in the process of the invention. Suitable sheetsilicates are kaolins, smectites, illites, bentonites(montmorillonites), hectorites, pyrophillites, attapulgites, sepiolitesand laponites, preferably bentonites, hectorites and attapulgites whichhave been ion-exchanged with a quaternized ammonium salt, particularlypreferably bentonite which has been ion-exchanged with a quaternaryammonium salt.

Bentonites, hectorites and attapulgites which have been ion-exchangedwith quaternized ammonium salts are commercially available:Quaternium-18 bentonite as Bentone 34 from Rheox Corp. and as Claytone34, Claytone 40 and Claytone XL from Southern Clay; stearalkoniumbentonite as Tixogel LG from United Catalysts, as Bentone SD-2 fromElementis Specialties and as Claytone AF and Claytone APA from SouthernClay; Quaternium-18/benzalkonium bentonite as Claytone GR, Claytone HTand Claytone PS from Southern Clay; Quaternium-18 hectorites as Bentone38 from Rheox Corp.; dihydrogenated tallow benzylmonium hectorites asBentone SD-3 from Rheox Corp.; stearalkonium hectorites as Bentone 27from Rheox Corp.; and also cationized attapulgite as Vistrol 1265 fromCimbar. These ion-exchanged sheet silicates can be used in the processof the invention either as powders or in the form of the commerciallyavailable dispersions in an oil or an organic solvent.

Apart from the commercial bentonites, hectorites and attapulgites whichhave been ion-exchanged with tetraalkylammonium ions, it is alsopossible to use the corresponding materials which have beenion-exchanged with quaternized alkanolamine fatty acid esters, inparticular bentonite which has been ion-exchanged withdimethyldiethanolammonium monoesters and diesters of fatty acids as wellas methyltriethanolammonium monoesters, diesters and triesters of fattyacids. Preference is given to using such esters with saturated fattyacids, in particular saturated fatty acids having from 12 to 18 carbonatoms.

In a preferred embodiment of the process of the invention, thewater-insoluble, cationized inorganic carrier material is arranged in afixed bed. Steps a) and b) of the process are then carried out bypassing the molybdate- or tungstate-containing aqueous solution througha fixed bed which contains the water-insoluble, cationized inorganiccarrier material. While the molybdate- or tungstate-containing aqueoussolution is being passed through the fixed bed, the molybdate ortungstate present in the solution is bound to the water-insoluble,cationized inorganic carrier material and the aqueous solution leavingthe fixed bed is depleted in molybdate or tungstate. After loading ofthe water-insoluble, cationized inorganic carrier material located inthe fixed bed, steps c) and d) of the process are carried out by passingan aqueous solution having a pH in the range from 6 to 14 through thefixed bed which has been laden with molybdate or tungstate in steps a)and b). The aqueous solution leaving the fixed bed here contains themajor part of the molybdate or tungstate which was bound to thewater-insoluble, cationized inorganic carrier material in step a) andthe fixed bed can, after carrying out these steps, once again be usedfor recovering molybdate or tungstate in steps a) and b).

The passage of the molybdate- or tungstate-containing aqueous solutionthrough the fixed bed is preferably stopped before the content ofmolybdate or tungstate in the aqueous solution leaving the fixed bedrises above the desired residual content.

The fixed bed preferably contains a water-insoluble filler material inaddition to the water-insoluble, cationized inorganic carrier materialin order to increase the porosity of the fixed bed. Suitablewater-insoluble filler materials are filter aids known from the priorart, which can be synthetic or natural, organic or inorganic in nature.A suitable inorganic filter aid is, for example, the silica gelobtainable under the trade name Celite 503 from Merck. A suitablenatural organic filter aid is, for example, cellulose which isobtainable under the trade name Jelucel HM 200 from Jelu. Syntheticpolymers of which the dewatering mats in the washing presses of pulp andpaper machines are made can also be used as water-insoluble fillermaterial. Particular preference is given to using cellulose aswater-insoluble filler material. The fixed bed preferably contains thewater-insoluble, cationized inorganic carrier material and thewater-insoluble filler material in a weight ratio of from 10:1 to 1:100.The fixed bed particularly preferably contains a composition containinga water-insoluble, cationized inorganic carrier material and cellulosein a weight ratio of from 10:1 to 1:100, in particular from 10:1 to1:10. The use of an additional filler material makes it possible toimprove the degree of loading of the water-insoluble, cationizedinorganic carrier material with molybdate or tungstate. In addition, thepressure drop during passage of the aqueous solutions through the fixedbed can be reduced and the process can be carried out more rapidly, andmalfunctions due to blockage of the fixed bed by impurities can beavoided.

Preference is given to using at least two fixed beds which are connectedin parallel and in which the steps a) and b) and the steps c) and d) arecarried out alternately, i.e. the recovery of molybdate or tungstatefrom an aqueous solution is carried out in steps a) and b) in a firstfixed bed while the molybdate or tungstate is detached again from thesupport in steps c) and d) in a second fixed bed which is connected inparallel and has already been laden with molybdate or tungstate. In aparticularly preferred embodiment, the fixed beds arranged in parallelare then switched over so that the passage of the molybdate- ortungstate-containing aqueous solution through a fixed bed occurscontinuously.

The invention additionally provides a process for recovering molybdateor tungstate in the delignification of pulp, in which the pulp isreacted in an aqueous mixture containing from 0.1 to 5% by weight ofhydrogen peroxide and from 10 to 1000 ppm of molybdenum in the form ofmolybdate or from 200 to 10 000 ppm of tungsten in the form oftungstate, in each case based on the mass of dry pulp, at a temperatureof from 30 to 100° C. and a pH in the range from 1 to 7 and the pulp isseparated from the aqueous solution, molybdate or tungstate is recoveredfrom the resulting aqueous solution by means of the above-mentionedsteps and the aqueous solution laden with molybdate or tungstate whichresults in the last step is recycled to the delignification of pulp withhydrogen peroxide.

In the delignification of pulp using molybdate or tungstate as catalyst,from 0.1 to 5% by weight, preferably from 0.5 to 4% by weight andparticularly preferably from 1 to 3% by weight, of hydrogen peroxide,based on the mass of dry pulp, is used. When molybdate is used ascatalyst, the molybdate is used in an amount of from 10 to 1000 ppm,preferably from 100 to 700 ppm and particularly preferably from 200 to600 ppm, of molybdenum, based on the mass of dry pulp. When tungstate isused as catalyst, the tungstate is used in an amount of from 200 to 10000 ppm, preferably from 500 to 5000 ppm and particularly preferablyfrom 1500 to 3000 ppm, of tungsten, based on the mass of dry pulp. Aneffective delignification and bleaching of the pulp is achieved and apulp having a reduced tendency to undergo yellowing is obtained bychoosing the amounts of hydrogen peroxide and molybdate or tungstate inthese ranges.

The delignification of pulp using molybdate or tungstate as catalyst iscarried out at a temperature of from 30 to 100° C., preferably from 60to 95° C. and particularly preferably from 75 to 95° C., with the pHbeing selected in the range from 1 to 7, preferably from 2 to 6 andparticularly preferably from 2.5 to 5.5. The choice of reactionconditions effects rapid and effective delignification and bleaching ofthe pulp. In addition, the delignification with addition of molybdate ortungstate can be combined with further process steps for delignificationand/or bleaching under these reaction conditions with only a smalladditional input of energy and/or chemicals for adjusting temperatureand/or pH.

The following examples illustrate the claimed process but withoutrestricting the scope of the invention.

EXAMPLES Example 1 (Comparative Example) Delignification of Pulp UsingHydrogen Peroxide without Addition of Molybdate

761 g of eucalyptus pulp, corresponding to 200 g of absolutely dry pulp,having a kappa number of 10.3, a brightness of 57.0% ISO and a yellowvalue of 22.1, were brought to a consistency of 10% by weight with waterand 1.0% by weight of hydrogen peroxide (based on absolutely dry pulp)and the pH was adjusted to 2.7 with sulphuric acid. The mixture washeated at 90° C. in a plastic bag in a waterbath for 120 minutes. Waterwas then added so as to give a suspension having a consistency of 2% byweight, and the pulp was filtered off on a suction filter using filterpaper. The pulp which had been treated in this way had a kappa number of5.4, a brightness of 60.2% ISO and a yellow value of 22.6. The filtrateobtained had a pH of 3.0. A hydrogen peroxide conversion of 59% wascalculated from the residual peroxide content of the filtrate.

Example 2 Delignification of Pulp Using Hydrogen Peroxide with MolybdateRecovery using Cationized Sheet Silicate

Example 1 was repeated but sodium molybdate was additionally added in anamount of 500 ppm of molybdenum, based on absolutely dry pulp, to themixture of pulp, water and hydrogen peroxide before heating. The treatedpulp had a kappa number of 3.5, a brightness of 61.5% ISO and a yellowvalue of 19.8. The filtrate obtained had a pH of 3.0. A hydrogenperoxide conversion of 79% was calculated from the residual peroxidecontent of the filtrate. The filtrate contained 9.8 ppm of molybdenum,which corresponded to 98% of the amount used.

The pH of the filtrate was adjusted to 3.7 with a 10% by weight sodiumhydroxide solution. 0.2% by weight (based on the weight of the filtrate)of cationically modified bentonite BENTONE® SD-2 (Elementis Specialties)was then added and dispersed by means of a dispersing stirrer providedwith a perforated disc (Pendraulik model LD50) at a rotational speed ofabout 1000 min⁻¹ for 15 minutes. The pH was once again adjusted to 3.7with a 10% by weight sodium hydroxide solution and the dispersion wasfiltered on a suction filter using filter paper. The molybdenum contentin the filtrate was 0.46 ppm, which corresponds to a removal ofmolybdenum by the bentonite of 95%.

The bentonite filtercake was sucked dry and stirred into water, whose pHhad been adjusted to 10 with a 10% by weight sodium hydroxide solution,at a consistency of 5% using a magnetic stirrer at 50° C. for 30minutes. The pH was once again adjusted to 10 with a 10% by weightsodium hydroxide solution, the mixture was filtered on a suction filterusing filter paper and the filtercake was washed twice using, in eachcase, 20% of the filtrate volume of water which had a temperature of 50°C. and whose pH had been adjusted to 10 with sodium hydroxide solution.The filtrate obtained contained 91% of the amount of molybdate used forthe treatment of the pulp.

Example 3 Delignification of Pulp using Recovered Molybdate

Example 2 was repeated using an amount of 228 g of eucalyptus pulp,corresponding to 60 g of absolutely dry material. However, the filtrateobtained at the end of Example 2 was used instead of sodium molybdatefor producing the mixture of pulp, water, hydrogen peroxide andmolybdate. Instead of fresh bentonite BENTONE® SD-2, the correspondingamount of the bentonite filtercake obtained at the end of Example 2 wasused.

The treated pulp had a kappa number of 2.9, a brightness of 62.2% ISOand a yellow value of 19.1. The filtrate obtained after the treatment ofthe pulp had a pH of 3.0. A hydrogen peroxide conversion of 81% wascalculated from the residual peroxide content of the filtrate. Thefiltrate contained 9.8 ppm of molybdenum, which corresponded to 98% ofthe amount used. After the treatment of the filtrate with the bentoniteat pH 3.7, the molybdenum content in the filtrate was 0.54 ppm, whichcorresponds to a removal of molybdenum by the bentonite of 94%. Thefiltrate obtained in the subsequent treatment of the bentonite withwater of a pH of 10 contained 90% of the amount of molybdate used forthe treatment of the pulp.

Example 4 (Comparative Example) Delignification of Pulp Using HydrogenPeroxide and Molybdate Recovery using Ion-Exchange Resin

Example 2 was repeated using eucalyptus pulp having a kappa number of12.0, a brightness of 52.3% ISO and a yellow value of 29.9. The filtrateobtained after the delignification of the pulp contained 8.2 ppm ofmolybdenum. To recover the molybdate, the same amount of anion-exchangeresin DOWEX M-43 was added in place of the cationic sheet silicate andthe mixture was stirred for 60 minutes using a magnetic stirrer in placeof the dispersing stirrer. The molybdenum content in the filtrate fromthe molybdate recovery was 2.0 ppm, which corresponds to a removal ofmolybdenum by the anion-exchange resin of 76%.

Compared to Example 2, Example 4 shows that the separation of molybdatefrom the filtrate of the delignification of pulp using the cationizedsheet silicate is more complete than when using the anion-exchangeresin.

Example 5 Recovery of Molybdate Using Different Sheet Silicates

Aqueous solutions of sodium molybdate which had a molybdenum content of30 ppm and had been adjusted to a pH of 3.7 using sulphuric acid were ineach case admixed with 0.2% by weight (based on the weight of thesolution) of sheet silicate and the sheet silicate was dispersed using adispersing stirrer provided with a perforated disc (Pendraulik modelLD50) at a rotational speed of about 1000 min⁻¹ for 15 minutes. The pHwas once again adjusted to 3.7 with a 10% by weight sodium hydroxidesolution and the dispersion was filtered on a suction filter usingfilter paper. In the case of the sheet silicate Syntal® 696, the pH hadto be adjusted by addition of sulphuric acid. Table 1 shows the sheetsilicates examined, the molybdenum content of the filtrate and thefilterability of the sheet silicate laden with molybdate.

The results in Table 1 show that the molybdate can be recovered morecompletely and, owing to the improved filterability, more simply usingthe cationized sheet silicate than when using an uncationized sheetsilicate.

Example 6 (Comparative Example) Recovery of Molybdate Using VariousIon-Exchange Resins

Example 5 was repeated using an ion-exchange resin in place of a sheetsilicate and a magnetic stirrer in place of the dispersing stirrer.Table 2 shows the ion-exchange resins examined, the content ofmolybdenum in the filtrate and the filterability of the ion-exchangeresin laden with molybdate.

Compared to the results in Table 1, the results in Table 2 show that themolybdate can be recovered more completely using the cationized sheetsilicate than when using an ion-exchange resin.

TABLE 1 Recovery of molybdate using different sheet silicates Molybdenumcontent in the Sheet silicate filtrate in ppm Filterability Bentone ®SD-2 (cationized less than 1 readily bentonite, Elementis filterableSpecialties) EX M 1694 * (acid- 24 readily activated Ca bentonite,filterable Süd-Chemie) Bentone ® HC * (purified not determined notfilterable hectorite, Elementis Specialties) Syntal ® 696 * (basic 5.6difficult to Mg—Al carbonate, filter Süd-Chemie) * not according to theinvention

TABLE 2 Recovery of molybdate using various ion-exchange resinsMolybdenum content in the Ion-exchange resin filtrate in ppmFilterability DOWEX ® M-43 7.9 readily (weakly basic anion- filterableexchange resin, Dow chemical) DOWEX ® PSR-3 14.6 readily (strongly basicanion- filterable exchange resin, Dow chemical) DOWEX ® NSR-1 13.4readily (strongly basic anion- filterable exchange resin, Dow chemical)DOWEX ® MARA MSA 13.9 readily (strongly basic anion- filterable exchangeresin, Dow chemical)

Example 7 Recovery of Molybdate Using a Fixed Bed of CationizedBentonite

2 g of cationically modified bentonite BENTONE® SD-2 (ElementisSpecialties) were suspended in 40 ml of water and filtered off on asuction filter having a diameter of 4.5 cm and provided with a paperfilter having a pore size of 12-25 μm. The filter cake obtained had aheight of about 5 mm and was used as fixed bed for the recovery ofmolybdate. For this purpose, 500 ml of a solution, which had beenobtained in the delignification of pulp and contained a silicomolybdatein an amount of 12.9 ppm of Mo, was drawn through the filter cake bysuction at room temperature in portions of 100 ml each and theconcentration of Mo in the portions obtained was subsequently determinedusing Merckoquant® molybdenum test strips. The first two portionscontained molybdate in an amount of less than 1 ppm of Mo, the next twoportions contained less than 5 ppm of Mo and only the last portioncontained molybdate in an amount of more than 5 ppm of Mo. Threeportions of 20 ml each of a 0.5% by weight sodium hydroxide solutionwere subsequently drawn through the filter cake by suction and thefiltrates obtained were combined. The combined filtrates containedmolybdate in an amount of more than 50 ppm of Mo, i.e. more than 65% ofthe molybdenum present in the solution obtained in the delignificationof pulp.

Example 8 Recovery of Molybdate Using a Fixed Bed of CationizedBentonite and a Filler Material

Example 7 was repeated with the difference that, to produce the fixedbed, first a suspension of 1.27 g of Celite® 503 (Merck) in 10 ml ofwater and subsequently a suspension of 2 g of Celite® 503 and 2 g ofBENTONE® SD-2 in 40 ml of water were drawn with suction through the samesuction filter. The same recovery of molybdate was achieved.

Example 9

Recovery of molybdate using a fixed bed of cationized bentonite and afiller material

Example 7 was repeated with the difference that, to produce the fixedbed, a suspension of 1 g of beech sulphite pulp, 1 g of Celite® 503 and2 g of BENTONE® SD-2 in 80 ml of water were drawn with suction throughthe suction filter and three portions of 50 ml each of a 0.5% by weightsodium hydroxide solution were used in place of three portions of 20 mleach of a 0.5% by weight sodium hydroxide solution. The same recovery ofmolybdate was achieved.

Example 10 Recovery of Molybdate Using a Fixed Bed of CationizedBentonite and Pulp

10 g of beech sulphite pulp and 5 g of cationically modified bentoniteBENTONE® SD-2 (Elementis Specialties) were dispersed in 500 ml of waterwith f a dispersing stirrer having a perforated disc (Pendraulik modelLD50) at a rotational speed of about 1500 min⁻¹ for 1 minute and thedispersion obtained was filtered with suction on a suction filter havinga diameter of 7 cm and provided with a paper filter having a pore sizeof 12-25 μm. The filter cake obtained was used as fixed bed for therecovery of molybdate. For this purpose, a solution which had beenobtained in the delignification of pulp and contained a silicomolybdatein an amount of 12.9 ppm of Mo was drawn through the filter cake bysuction in portions of 100 ml each and the concentration of Mo in theportions obtained was subsequently determined using Merckoquant®molybdenum test strips. The first 7 portions contained molybdate in anamount of less than 1 ppm of Mo, the next 4 portions contained less than5 ppm of Mo and only the last portion contained molybdate in an amountof more than 5 ppm of Mo. Three portions of 50 ml each of a 0.5% byweight sodium hydroxide solution and then 50 ml of water weresubsequently drawn through the filter cake by suction and the filtratesobtained were combined. The combined filtrates contained molybdate in anamount of more than 50 ppm of Mo, i.e. more than 75% of the molybdenumpresent in the solution obtained in the delignification of pulp.

1. A process for bleaching pulp comprising: a) reacting said pulp withhydrogen peroxide at a temperature of from 30 to 100° C. and at a pH inthe range from 1 to 7, wherein said reaction is performed in an aqueousmixture comprising: i) from 0.1 to 5% by weight of said hydrogenperoxide and ii) from 10 to 1000 ppm of molybdenum in the form ofmolybdate or from 200 to 10 000 ppm of tungsten in the form oftungstate, in each case based on the mass of dry pulp; b) separating thepulp bleached with hydrogen peroxide in step a) from said aqueousmixture to obtain an aqueous solution comprising molybdate or tungstate;c) contacting the aqueous solution containing molybdate or tungstateproduced in step b) with a water-insoluble, cationized inorganic carriermaterial at a pH in the range from 2 to 6 to give a carrier materialladen with molybdate or tungstate and an aqueous solution depleted inmolybdate or tungstate; d) separating said carrier material laden withmolybdate or tungstate from said aqueous solution depleted in molybdateor tungstate; e) contacting said carrier material laden with molybdateor tungstate with a second aqueous solution at a pH in the range from 6to 14 to give a carrier material depleted in molybdate or tungstate andan aqueous solution comprising molybdate or tungstate, and f) separatingsaid carrier material depleted in molybdate or tungstate produced instep c) from said aqueous solution comprising molybdate or tungstate;wherein said cationized inorganic carrier material is a cationized sheetsilicate selected from the group consisting of bentonites, hectoritesand attapulgites and said sheet silicate has been ion-exchanged with aquaternary tetraalkylammonium salt or with a quaternized alkanolaminefatty acid ester.
 2. The process of claim 1, further comprisingrecycling the aqueous solution comprising molybdate or tungstateproduced in step f) to step a).
 3. The process of claim 1, wherein thewater-insoluble, cationized inorganic carrier material used to contactthe aqueous solution containing molybdate or tungstate in step c)comprises, at least in part, the carrier material depleted in molybdateor tungstate from step f).
 4. The process of claim 1, wherein theaqueous mixture of step a) comprises from 0.5 to 4% by weight ofhydrogen peroxide, based on the mass of dry pulp.
 5. The process ofclaim 1, wherein the aqueous mixture of step a) contains from 100 to 700ppm of molybdenum, based on the mass of dry pulp, in the form ofmolybdate.
 6. The process of claim 1, wherein the aqueous mixture ofstep a) comprises from 500 to 5000 ppm of tungsten, based on the mass ofdry pulp, in the form of tungstate.
 7. The process of claim 1, whereinthe pH in step c) is in the range from 3 to
 5. 8. The process of claim1, wherein the pH in step e) is in the range from 8 to
 12. 9. Theprocess of claim 1, wherein the cationized sheet silicate is abentonite.
 10. The process of claim 1, wherein the cationized inorganiccarrier material is used in an amount of from 10 to 1000 parts by weightof carrier material per part by weight of molybdenum or in an amount offrom 200 to 10 000 parts by weight of carrier material per part byweight of tungsten in the contacting with an aqueous solution containingmolybdate or tungstate in step c).
 11. The process of claim 10, whereinthe cationized inorganic carrier material is used in an amount of from50 to 500 parts by weight of carrier material per part by weight ofmolybdenum.
 12. The process of claim 10, wherein the cationizedinorganic carrier material is used in an amount of from 1000 to 5000parts by weight of carrier material per part by weight of tungsten. 13.The process of claim 1, wherein steps c) and d) are carried out bypassing the aqueous solution containing molybdate or tungstate through afixed bed comprising the water-insoluble, cationized inorganic carriermaterial and steps e) and f) are carried out by passing an aqueoussolution having a pH in the range from 6 to 14 through the fixed bedwhich has been laden with molybdate or tungstate in steps c) and d). 14.The process of claim 13, wherein the fixed bed comprises awater-insoluble filler material in addition to the water-insoluble,cationized inorganic carrier material.
 15. The process of claim 14,wherein cellulose is used as waterinsoluble filler material.
 16. Theprocess of claim 14, wherein the water-insoluble, cationized inorganiccarrier material and the water-insoluble filler material are used in aweight ratio of from 10:1 to 1:100.
 17. The process of claim 13, whereinat least two fixed beds connected in parallel are used, in which stepsc) and d) and steps e) and f) are carried out alternately.
 18. Theprocess of claim 17, wherein the passing of the aqueous solutioncontaining molybdate or tungstate through a fixed bed is carried outcontinuously by switching over between fixed beds connected in parallel.